Objectives: 1. To characterize the endogenous life cycle of Toxoplasma in cats. 2. To study toxoplasmosis in cats and in models for the human. 3. To study immunity to toxoplasmosis as shown by absent disease manifestations and limitation of oocyst excretion by cats. 4. To devise methods of preventing toxoplasmosis as a disease in humans, including the production of vaccines for cats and for humans. Cats will be infected with oocysts for study of the endogenous cycle. The nature of immunity will be studied after oral and parenteral infection, and oral challenge. A large number of strains of Toxoplasma will be collected for sero- and immunotyping. Strains suitable for vaccination will be selected on the basis of immunogenicity, safety and effectivity. A vaccine will be considered on the basis of stability, effectiveness and safety.